The present invention relates to a film guide, particularly for film viewers, editors, projectors or the like, which includes a window for defining the edges of the image and defining the position of the film, the opening width of the window transverse to the film transporting direction being smaller than the width of the film transporting path. Such windows have a defined position with respect to the lens and cause the window plane or the film contacting it, respectively, to be reproduced in sharp focus on the viewing or projection screen.
When dimensioning the lens of a projector, the fact must be considered that the transported film has an unavoidable curvature which is caused by the layer bearing the image. Particularly with slides, where the light acts longer on one picture, the heat emanating from the light source acts on the film for a corresponding length of time and produces an additional curvature or a change in the depth of the curvature.
In order to assure a sharp projected or viewed image, the lenses in known devices are dimensioned to take into account the largest occurring curvature of the film, in order to provide a great depth of focus for the varying curvature of quickly moving slide film. The result of this is that, particularly with projectors, the projected image is relatively small compared to the distance of the projector from the screen, due to the large focal length required to obtain the great depth of focus. Despite the substantial adaptation resulting from making the focal length of the lens correspond to the greatest occurring curvature of the film, the focus must often be readjusted in conventional projectors presently available because with slides which are not glass, the maximum occurring curvature may be even greater than provided for, depending on the quality of the framing job.